This invention relates to polysaccharide esters containing aldehyde groups, to their preparation from the corresponding acetals, and to a guanidine derivative used for the preparation of one of the acetals. It also relates to the use of the aldehydes as wet strength agents in paper.
As used herein, the term "paper" includes sheet-like masses and molded products made from fibrous cellulosic materials which may be derived from natural sources as well as from synthetics such as polyamides, polyesters, and polyacrylic resins, and from material fibers such as asbestos and glass. In addition, papers, made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is also included within the term "paper".
Oxidative and non-oxidative methods have been used to introduce aldehyde groups into polysaccharides such as starches, gums, and celluloses. The oxidative methods used have included treatment with periodic acid, periodates, or alkali metal ferrates. See U.S. Pat. No. 3,086,969 (issued Apr. 23, 1963 to J. E. Slager) which discloses an improved process for the preparation of a dialdehyde polysaccharide, (e.g., starch) using periodic acid; U.S. Pat. No. 3,062,652 (issue Nov. 6, 1962 to R. A. Jeffreys et al.) which discloses the preparation of dialdehyde gums (e.g., gum acacia, pectin, and guar) using periodate or periodic acid; and U.S. Pat. No. 3,632,802 (issued Jan. 4, 1972 to J. N. BeMiller et al.) which discloses a method for oxidizing a carbohydrate, (e.g., starch or cellulose) with an alkali metal ferrate, which selectively oxidizes the primary alcohol group on the side chains without ring cleavage or oxidation of the ring hydroxyls.
The disadvantages of the oxidative method include degradation to lower molecular weight products and the formation of carboxyl groups due to further oxidation of the aldehyde groups. U.S. Pat. No. 3,553,193 (issued Jan. 5, 1973 to D. H. LeRoy et al.) describes an improved method for oxidizing starch using an alkali metal bromite or hypobromite under carefully controlled conditions, which reportedly results in a substantially greater proportion of carbonyl groups (i.e., aldehyde groups) than carboxyl groups.
The non-oxidative methods typically involve the reaction of the polysaccharide with an aldehyde-containing reagent. See U.S. Pat. No. 3,519,618 (issued July 7, 1970 to S. M. Parmerter) and U.S. Pat. No. 3,740,391 (issued June 19, 1973 to L. L. Williams et al.) which cover starch ether derivatives and U.S. Pat. No. 2,803,558 (issued Aug. 20, 1957 to G. D. Fronmuller) which covers a gum ether derivative. Water-soluble cellulose ethers (e.g., hydroxyethylcellulose) have also been reacted with glyoxal or ureaformaldehyde to give aldehyde-containing derivatives.
One of the disadvantages of introducing the aldehyde groups directly using an aldehyde-containing reagent is the possibility of the derivative reacting prior to use, e.g., crosslinking. This is a particular disadvantage when the products are being used to impart temporary wet strength to paper via a crosslinking reaction with the cellulose fibers. This prior reaction of the aldehyde functionality is also a problem when preparing crosslinked products in that it makes control of the final degree of crosslinking very difficult or impossible and it leads to a decrease in the amount of aldehyde functionality in the final product.
The preparation of starch esters from acyl guanidines is disclosed in U.S. Pat. No. 3,728,332 issued Apr. 17, 1973 to M. M. Tessler et al. The acylguanidines used do not contain acetal groups.
A method for introducing aldehyde groups into polysaccharides is described in Ser. No. 758,634 filed July 24, 1985. The first step involves the reaction of the polysaccharide with a reagent containing an acetal-functionality and an additional group reactive with the polysaccharide to form an ether linkage. The second step involves conversion of the acetal to the aldehyde at a pH of 7 or less. The etherification reaction must be carried out under alkaline conditions (e.g., pH of &gt;7). Under these conditions other substituents present on the polysaccharide (e.g., ester, carbonyl, or epoxide) may be unstable and hence removed.
Therefore, there is a need for aldehyde-containing polysaccharides, particularly for a method for preparing them in the presence of alkali-labile substituent groups.